Method for abatement of voc in exhaust gases by wet pulse corona discharge

ABSTRACT

A method for abatement of volatile organic compounds (VOC) in an exhaust gas stream is disclosed. That method comprises passing an exhaust gas stream ( 40 ) through a pulsed corona discharge chamber ( 10 ) in the presence of flowing water ( 30 ) to form one or more oxidation products that dissolve in the water and provide an effluent water stream ( 42 ) containing the oxidized VOC and an effluent gas stream ( 32 ) having a deleted amount of VOC. The pulsed corona discharges at a rate of about 0.01 to about 2 kHz. The ratio of the water spray rate to the exhaust gas flow is about 0.2 milliliters/minute at one standard liter per minute of exhaust gas flow, and an expenditure of not more than 50 eV per molecule of VOC is utilized. The method provides a destruction and removal efficiency of about 90 percent or more.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional application Ser.No. 60/367,231 that was filed on Mar. 25, 2002.

GOVERNMENTAL SUPPORT

The invention was made in the frame of a DOE sponsored project, ProjectIdentification No. DE-FC07-00ID13868. The government has certain rightsin the invention pursuant to that support.

DESCRIPTION

Low-temperature, non-equilibrium plasmas (also called “non-thermal”plasmas) are an emerging technology for abating diluting volatileorganic compounds (VOC) emissions. These plasmas can be produced by avariety of electrical discharges or electron beams.

The basic feature of plasma technologies is that they produce plasma inwhich the majority of the electric energy (more than 99%) goes intoproduction of energetic electrons, instead of heating the entire gasstream. These energetic electrons produce excited species, free radicalsand ions) as well as additional electrons through the electron impactdissociation, excitation and ionization of the background molecules.These excited species, in turn, oxidize, reduce, or decompose thepollutant molecules.

This mechanism of VOC removal is in contrast to the mechanism involvedin thermal processes (such as plasma torches or furnaces, regenerativethermal oxidation (RTO) and several chemical techniques) that requireheating the entire gas stream in order to destroy pollutants. Inaddition, the low-temperature plasma technology is highly selective andhas relatively low maintenance requirements. Its high selectivityresults in relatively low energy costs for emissions control while lowmaintenance keeps annual operating expenses low. Furthermore, theseplasma discharges are very uniform and homogeneous (except glidingarcs), which results in high process productivity.

As mentioned above, RTO is more effective at high concentrations of VOC.Plasma is normally used at lower concentrations of VOC and in the caseswhere combustion is not effective, such as removal of nitrogen oxides(NO_(x)) or sulfur-containing compounds. Scrubbing of exhaust gases withwater might be required for both techniques, but some plasma reactorscan be designed as a single unit with a scrubber to reduce the equipmentcost.

The gliding arc is a high-pressure gas discharge with high electrontemperature and density and has been proposed for chemical gasprocessing [Lesueur et al., J. de Physique 51: C557-C564, 1990]. The arcstarts in a narrow gap between two or more diverging electrodes in a gasflow when the electric field in this gap reaches approximately 3 kV/mmin air [Raizer, Gas Discharge Physics, Berlin, Springer-Verlag, 1997].Then the arc current increases very rapidly, and the voltage on the arcdrops. If the gas flow is strong enough, it forces the arc to move alongthe diverging electrodes and to elongate.

The growing arc demands more power to sustain itself. At the moment whenits resistance becomes equal to the total external resistance, thedischarge consumes one-half of the power delivered by the source. Thisis the maximum power that can be transferred to the arc from theconstant-voltage power supply. Next, the length continues increasing,but the supplied power is insufficient to balance the energy lost inheat transfer to the surrounding gas. The arc cools down and finallyextinguishes. The next cycle starts immediately after the voltagereaches the breakdown value, usually just after the fading of theprevious arc. (If the voltage is high enough, and the gap is verynarrow, a new arc starts even before extinguishing of the old one[Pellerin et al., J. Physics D-Applied Physics, 33(19): 2407-2419,2000]. A typical repetition rate of the arc is in the range from 10 Hzto 100 Hz and changes with the gas flow rate: the higher is the flowrate, the higher is the frequency.

Unlike regular high-current thermal arcs where both gas and electrontemperatures (T and T_(e)) could be as high as 10000K, the low-current(from 0.1 to 1 A) gliding arcs are believed to operate innon-equilibrium regimes (T<T_(e)), and have low gas temperatures. Thearc voltage could be as high as several kilovolts. Typical vibrationaland translational temperatures of the gas in a low-temperature glidingarc were measured to be respectively about 2000-3000K and 800-2100 K[Czernihowski et al., Acta Physica Polonica A, 89: 595-603, 1996].Similar features are also intrinsic for microwave discharges; showingthat low-current gliding arcs could be definitely useful for plasmachemistry.

The physics of the gliding arc evolution is not well understood,especially the development of non-equilibrium conditions and stabilityat diminishing currents. Older models [Mutaf-Yardimci et al., J. AppliedPhysics, 87: 1632-1641, 2000] did not account for variation of plasmaconductivity and temperature during the gliding arc progress. Thisapproach did not explain some recently discovered non-equilibriumeffects in the low-current gliding arcs.

Mario Sobacchi [Sobacchi, M. G., “Hydrocarbon Processing inNon-equilibrium Gas Discharges”, Master of Science Thesis, University ofIllinois at Chicago, 2001] investigated the possibility of removal ofspecific chemical compounds with a low-current gliding arc discharge. Atypical value of energy consumption for this discharge was 1 kW·hr/m³,when the targeted value was 0.02 kW·hr/m³. It was noted thatnon-uniformity of the discharge imposes certain difficulties to treatthe gas efficiently. Experiments were performed using three differentexternal resistance values; i.e., 25, 50, and 100 kW. The output voltageof the power supply was 10 kV, when the air flow rate ranged from 20 to80 SLM.

Methanol (100 to 1000 ppm) was used as a substance to demonstrateviability of the gliding arc technology. In some runs, water was sprayedalong the flow direction. It was found that at 20 SLM of air andindependently of VOC concentration, all methanol was removed from thestream at power consumption of 0.2 kW·hr/m³. No organic byproducts weredetected by gas chromatography in the exhaust stream. The results wereinterpreted as encouraging, showing the possibility for furtherreduction of power consumption. Unfortunately, NO_(x) was detected inthe exhaust at level of 3000 ppm, much exceeding all reasonable limits.It could be expected, however, that for more non-equilibrium arcs withlower currents or higher flow rates of air, the temperature of the arcchannel will be lower, and reasonable levels of NO_(x) will be achieved.

Corona is a self-sustained electrical gas discharge that occurs onlywhen the electric field is sharply non-uniform. The field near one orboth the electrodes must be stronger than in the rest of the gap. Thissituation typically arises when the characteristic size r of theelectrode to which the high voltage is applied, is much smaller than theinter-electrode distance d. If one considers a wire-into-cylinderconfiguration, the electric field in the space between the coaxialcylinders of radii r (internal cylinder) and R is given as a function ofthe radial coordinate x as:$E = \frac{V}{\left\lbrack {x\quad{\ln\left( \frac{R}{\gamma} \right)}} \right\rbrack}$where V is the voltage between the cylinders.

There is always some ionization in atmosphere due to high-energyparticles coming from space. Existing electrons are accelerated by theelectric field. They can ionize more molecules of the gas. New electronsare accelerated in the field, and so forth. Corona discharge occurs onlywhen the value of the maximum electric field exceeds the breakdownfield. For coaxial cylinders, the critical electric field of coronaignition in air is expressed by an empirical formula suggested by Peek(1929):$E_{c} = {31\quad{{\delta\left( {1 + \frac{0.308}{\sqrt{\delta\quad r}}} \right)}\quad\left\lbrack {{kV}\text{/}{cm}} \right\rbrack}}$

-   -   where δ is the ratio of air density to the density in standard        pressure and temperature. It is clear that a constraint is        imposed on the maximum field, which occurs on the surface of the        sharp electrode, that is the high field or “corona carrying”        electrode.

The formulae given above describe fairly well the physics of the coronawith DC or slowly changing fields, such as in gas phase corona reactors(GPCR). For these applications, to be sustained at non-equilibriumstate, the discharge can acquire only small power. Higher power wouldcause sparking and eventually lead to thermal arc formation.

Natural power limitations of DC corona technology gave rise todevelopment of the pulsed corona technology. In a pulsed coronadischarge, voltage is applied to the sharp electrode as a series of fastrising pulses. Incoming pulses generate a number of microdischarges(streamers); the faster the pulse, the more streamers are produced perunit length, increasing the power input per unit gas volume.

The width of the applied pulse must be minimized in order to avoidformation of spark discharge or thermalization of the streamers. Formost geometries, then, the pulse rise time must be on the order of fewnanoseconds and the duration of the pulse on the order of 100 ns.

The electron energy, in fact, depends on both the intensity of theelectric field (i.e. on the voltage) and the mean free path. For a fastrising pulse, the mean electric field is very high (since the peakvoltage is high), allowing the electrons to gain enough energy for thedischarge to take place; however, since the duration of each pulse ismuch smaller than the interval between pulses, the mean required poweris low (e.g., it is in the order of few Watts for the setups used forthese experiments) even though the actual power of each single pulse isvery high (about 1 MW). The negative polarity is predetermined by theexisting equipment.

DC corona is a uniform discharge. In contrary, the pulsed corona ishighly non-uniform. All the discharge power is localized in streamers.To describe the physics of the discharge, one must consider a phenomenonof a streamer as a whole. Electromagnetic model of physics in streamersmust be coupled with chemistry of excited species, and with internalparticles (electrons, photons) transport. This coupling is verydifficult because of enormous amount of computational work. Simplifiedmodels were developed, but they are still not adequate for the purposesof trustworthy chemistry prediction. Many parameters are known withgreat degree of uncertainty, such as electron cross-sections.

There are numerous methods for the waste gases treatments from volatileorganic compounds (VOC) with the help of electrical discharges, andparticularly with the help of the pulsed corona discharge [Malik et al.,Chinese J. Chem. Eng., 7(4) (1999)]. To remove one large VOC moleculewith the help of such methods it is necessary to produce several activeradicals like OH. The energy price of one radical production is veryhigh—about 50 eV per radical. As a result, total price of one VOCmolecule removal (transformation of VOC molecule into H₂O and CO₂) isalso very high—about 300 eV. It is possible in this case to satisfy theindustry demand to spend not more than about 10 W-Hour/m³ of the wastegas only if VOC concentration is not higher than about 30 ppm. Realindustrial waste streams such as that of the papermaking, metal cleaningand plating, plastics manufacture and the like industries have VOCcontamination levels that are several times higher (e.g., about 100 toabout 6000 ppm), so usual plasma methods for VOC removal are notapplicable . . . The invention described hereinafter provides one methodby which the latter, more highly loaded exhaust gas streams can beabated of VOC.

BRIEF SUMMARY OF THE INVENTION

The present invention contemplates a method for treating exhaust gasstreams that contain one or more volatile organic compounds (VOC) toabate those VOCs. A contemplated method for abatement of VOCs in anexhaust gas stream comprises passing an exhaust gas stream containingabout 60 to about 6000 ppm VOC, and more usually about 200 to about 3000ppm, through a pulsed corona discharge chamber in the presence of aspray of water droplets or a flowing film of water to form one or moreoxidation products that dissolve in the water spray droplets or film andprovide an effluent water stream and an effluent gas stream. In themethod, a contemplated pulsed corona discharges at a rate of about 0.01to about 2 kHz. The ratio of the water spray rate to the exhaust gasflow is about 0.2 to about 2 milliliters/minute at one standard literper minute of exhaust gas flow, and an expenditure of not more than 50eV per molecule of VOC is utilized. This method provides a destructionand removal efficiency of about 90 percent or more, thereby abating theVOC. The method can operate at substantially any temperature with theunderstanding that lower temperatures typically provide slower reactionrates.

In some preferred embodiments, the method is utilized in part of a wastetreatment system in which the relatively small amount of water thatcontains the dissolved oxidation products produced in this method isadmixed with the relatively larger volume of industrial waste waterproduced by papermaking, metal cleaning or plating process or plasticsmanufacturing process. In another preferred embodiment, the method isutilized after the gas phase is contacted with water as a mist ofparticles about 0.1 to about 1 millimeter in diameter or liquid water todissolve and thereby capture water-soluble VOC such as methanol andacetone. The effluent gas from such a pretreatment is then passed intothe wet pulsed corona discharge process discussed above with the liquidphase being further subjected to bioremediation or another type ofpurification treatment. The gas phase produced from the above process ispassed over a gaseous sorbent (absorbent and/or adsorbent) system suchas activated charcoal and vented to the air or passed into an air streamof an incinerator or the like to eliminate vestigial VOC that may haveescaped elimination in a previous step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic representation of the pulsed wet coronasystem of the present invention.

FIG. 2 is a graphical representation of experimental results fordestruction and removal efficiency (DRE) with using methanol as a modelVOC in which percentage DRE is shown on the ordinate and water flow rate[milliliters per minute ml/min)] is shown on the abscissa.

FIG. 3 is a graphical representation of experimental results for DREusing acetone as a model VOC, in which results are depicted as in FIG.2.

FIG. 4 is a graphical representation comparing the annualized costs inthousands of dollars (k$) versus plasma processing costs(watt-hours/cubic meter) for the present invention and a regenerativethermal oxidizer (RTO) of the prior art.

The present invention has several benefits and advantages.

One benefit is that high levels of VOC elimination efficiency can beobtained at relatively low start-up and running costs.

An advantage of the invention is that the products produced by thepulsed corona discharge reaction on the VOC are typically more solublethan are many of the VOC so that those materials can be more readilydissolved in the water flow that is used.

Another benefit of the invention is that the method and apparatusutilized are adaptable to being utilized with other air and waterpurification systems or methods.

Still further benefits and advantages of the invention will be apparentform the discussion that follows.

DETAILED DESCRIPTION OF THE INVENTION

A new method has been developed that combines the corona discharge withthe scrubber to provide a “wet” corona discharge. [Fridman et al.,199^(th) Meeting of the Electrochemical Society, Washington, Mar. 25-29,2001; Meeting Abstracts, Vol. 2001-1, Abstract No. 196; and Sobacchi etal., 15^(th) International Symposium on Plasma Chemistry, Orleans, Jul.9-13, 2001, Symposium Proceedings, Vol. VII: Poster Contributions, pp.3135-3140; Sobacchi, “Hydrocarbon Processing in Non-Equilibrium GasDischarges”, Master of Science Thesis, University of Illinois atChicago, May 16, 2001; Kalashnikov, “Non-Equilibrium Gliding Arc AndCorona Discharges For Abatement Of Volatile Organic Compounds”, DoctoralThesis, University of Illinois at Chicago, Jun. 26, 2002.]

To obtain desirable VOC destruction with low energy consumption it isnecessary to make proper design of pulsed corona discharge and scrubber.This method is applicable to substantially any existing waste gas streamthat emanates from an industrial site such as a metal cleaning andplating, paint manufacturing, plastics manufacturing, petroleum refiningand dye-making site that also includes a wastewater treatment facilityfor treating a waste water stream from a source other than the pulsedcorona discharge.

The present invention contemplates a method for treating exhaust gasstreams that contain one or more volatile organic compounds (VOC) toabate those VOCs. Illustrative exhaust gas streams include, withoutlimitation, those streams produced by papermaking, metal cleaning andplating, paint manufacturing, plastics manufacture, petroleum refining,dye-making and the like industries. In particular, a contemplated methodis particularly useful in abating VOC produced in papermaking processessuch as brownstock and oriented strandboard production (whose VOC aresimilar to those of papermaking), and particularly from the ventsstreams and dryer exhausts used in those processes.

A contemplated method for abatement of VOCs in an exhaust gas streamcomprises passing an exhaust gas stream containing about 60 to about6000 ppm VOC through a pulsed corona discharge chamber in the presenceof a spray of water droplets or a flowing film of water to form one ormore oxidation products that dissolve in the water spray droplets orfilm and provide an effluent water stream and an effluent gas stream. Inthe method, a contemplated pulsed corona discharges at a rate of about0.01 to about 2 kHz, and more preferably about 0.1 to about 1 kHz. Theratio of the water spray or flow rate to the exhaust gas flow is about0.2 to about 2 milliliters/minute at one standard liter per minute ofexhaust gas flow, and an expenditure of not more than 50 eV per moleculeof VOC is utilized. This method provides a destruction and removalefficiency of about 90 percent or more, to about 99 percent or more,thereby abating the VOC.

The method can operate at substantially any temperature with theunderstanding that lower temperatures typically provide slower reactionrates, as is usually the case for chemical reactions. More typicaltemperatures of the exhaust gas stream are about 103° F. (about 40° C.)to about 150° F. (about 65° C.). Such temperatures are determined priorto entry into the pulsed corona discharge apparatus.

More usual and preferred VOC levels are about 200 to about 4200 ppm.More preferably still, those VOC levels are about 300 to about 3000 ppm.

Specific values of the water spray or flow rate and the exhaust gas flowvolume can differ from one apparatus and gas stream to another. However,the ratio of water spray or flow rate to the exhaust gas flow isrelatively constant at about 0.2 to about 2 milliliters/minute at onestandard liter per minute (SLM) of exhaust gas flow. Thus, for wetcorona discharge systems larger than that used illustratively herein,the water spray or flow rate can be on the order of gallons, liters orkiloliters per minute, with an accompanying increase in gas stream flowso that the ratio of the two corresponds to that noted above.

The contemplated method is illustrated herein in relation to apapermaking facility. The illustrative target exhaust gas streams forthis invention such as high volume low concentration (HVLC) brownstockwasher vent emissions can contain various VOCs in varyingconcentrations, as are seen in Table I, below. TABLE I HVLC Brownstock“worst case scenario” Washer Vent HVLC Brownstock Washer Emitted VOCEmissions Vent Emissions component according NCASI* suggested by GP**Dimethyl Disulfide 2 ppm 20 ppm Dimethyl Sulfide — 1727 ppm Methanol 83ppm 2330 ppm Acetone 3 ppm — Terpenes 209 ppm 62 ppm Process ConditionsTemperature 103° F. 150° F. Relative Humidity 100% 100%*NCASI = National Council for Air and Stream Improvement**GP = Georgia Pacific

In the simplest case (shown schematically in FIG. 1) the wet coronasetup permits for a water film to flow on the internal walls of thereactor. In a larger scalable geometry a water spray can be used insteadof water film. Thus, the flow of water is scalable based on the size ofthe apparatus used, the VOC present in the exhaust gas, the volume ofgas flow and other known or readily determined parameters that areassociated with Henry's law and the equations discussed hereinafter.

While the present invention is susceptible of embodiment in variousforms, there is shown in the drawings a number of presently preferredembodiments that are discussed in greater detail hereafter. It should beunderstood that the present disclosure is to be considered as anexemplification of the present invention, and is not intended to limitthe invention to the specific embodiments illustrated.

Referring to the drawings, a wet pulse corona discharge system 10 isschematically represented in FIG. 1. That figure shows a reactor 12having internal walls 12 a over which a water film is permitted to flow.It will be understood by persons having skill in the art that in alarger scalable geometry a water spray can be used instead of a flow,without departing from the novel scope of the present invention. Acorona discharge wire 14 is provided within reactor 12. The reactor 12and wire 14 create a wire-into-cylinder coaxial electrode system 16. Theinner, high voltage electrode 14 is, in a preferred embodiment, a 0.5 mmdiameter Inconel® wire. The Inconel® alloy is selected for its superiorheat and oxidation resistance. It will, however, be understood thatother alloys or single metallic element wires, can be used withoutdeparting from the novel scope of the present invention.

The reactor vessel 12 creates an external electrode 18, made up, in apreferred embodiment of the present invention, of a 60 cm longcylindrical glass tube 20 with an internal diameter 22, in a preferredembodiment the diameter is 22.2 mm, surrounded by a sheet of perforatedmetal 23. The internal wall 20 a of the glass tube 20 is covered by alayer of absorbing material 24, which forms a uniformly distributedwater film in the reactor 12.

The tube 20 is held, in a preferred embodiment, at the top by a Teflon®holder (not shown) and at the bottom by a stainless steel holder (notshown); stainless steel is preferred for the bottom of the reactor toprevent deterioration of the holder itself due to the byproductsexpected in the liquid phase, such as sulfuric acid. It will beunderstood that different materials can be utilized, both for the topand the bottom, without departing from the novel scope of the presentinvention. Further, it will be understood that either one or both of thematerials noted above can be used either together or in othercombinations without departing from the novel scope of the presentinvention. Preferably, materials having similar qualities, such asnon-sticking and oxidation and reactivity free materials will be usedfor the materials of the top and bottom of the reactor.

The reactor 12 is sealed, in a preferred, by means of silicon O-rings(not shown). Although silicone O-rings are used in a preferredembodiment, it will be understood that O-rings of other materials,including rubber and plastic, can be used without departing from thenovel scope of the present invention. The top holder (not shown)comprises the connections of the lines, which are schematically shown inFIG. 1, for the incoming water 30 and the outgoing (effluent) gas 32. Ina preferred embodiment, water is introduced into the reactor 12 throughsixteen, equally spaced holes with a diameter of 1/32″ (about 0.8 mm),to provide a uniform injection around the circular tube. It will beunderstood that any number of equally spaced holes, of various suitablediameters (either uniformly of the same diameter or of differentdiameters with suitable results) can be used to provide a uniforminjection of water, without departing from the novel scope of thepresent invention.

The top holder (not shown) also contains the connection for theinternal, high voltage electrode 14 and for the high voltage probe(shown schematically in FIG. 1). The bottom holder (not shown) includesthreaded holes for the gas inlet 40 and for the water outlet (effluent)42 (shown schematically in FIG. 1). To avoid any contact between thismetal holder (not shown) and the high voltage (HV) electrode 14 and toprevent the formation of sparks, the wire 14 can be fixed to the holder(not shown) through a Teflon® insert (not shown), which completelycontains the lower portion of the wire. As noted above, the type ofinsert described is a preferred embodiment and is not meant to limit thescope of the present invention.

Power is supplied, in a preferred embodiment, to the plasma source by athyratron-based power supply 50, with 100 kΩ internal resistance andno-load voltage values from 0 to 20 kV. It will be understood thatsimilar power supplies, having desirable power producing capabilities,can be utilized without departing from the novel scope of the presentinvention. In the preferred embodiment, pulses of about 100 ns durationand 10 ns rise time are applied and are transmitted to the central wireelectrode 14 by means of a 50 Ω high-voltage, coaxial cable 52. Thepower of the plasma source is varied from 1 to 20 W by controlling thehigh voltage pulse amplitude and the pulse frequency, which could rangefrom 0.01 to 2 kHz. Voltage is measured using a high voltage probe (notshown) such as model P6015A 1000X by Tektronik Inc., currentmeasurements are performed, in a preferred embodiment, using a standardcurrent monitor such as current monitor #411 by Pearson Inc.

In the operation of the reactor 12 of the present invention, anelectrode 14 is placed within glass tube 20, water is input into thesystem in such a manner that a film of water forms on the internal walls12 a of the reactor 12, simultaneously an exhaust gas stream containingVOC is introduced and the electrode is charged. The result includes theoxidation of VOC, their dissolution in the flowing water effluent andthe release of gas phase byproducts in the gas effluent.

Correct choice of parameters should be based on theoretical estimationsor experimental results. The solubility of a gas in water is describedby the Henry's law, which states a relation between the concentrationsof a gas in the liquid and in the gas phases. Usually, the Henry's lawconstant k_(H) is defined as: $\begin{matrix}{k_{H} = \frac{c_{a}}{p_{g}}} & (1)\end{matrix}$

Here, c_(a) is the concentration of a species in the aqueous phase andp_(g) is the partial pressure of that species in the gas phase. Thecommonly used unit for the Henry constant is: $\begin{matrix}{\left\lbrack \frac{M}{atm} \right\rbrack = \left\lbrack \frac{{mol}_{aq}/m_{aq}^{3}}{atm} \right\rbrack} & (2)\end{matrix}$

The higher the value of the Henry's law constant for a given compound,the higher the solubility of that compound in water. Values of theHenry's law constants for the VOCs of interest are shown in Table II,below. Oxidized compounds such as methanol, acetone and the intermediatebyproducts from the oxidation of dimethylsulfide and α-pinene (that arethe main substances from terpenes in HVLC brownstock washer ventemissions) appear to have a much higher solubility than the non-oxidizedspecies. TABLE II Henry's law constant values in standard conditionsVolatile Organic Compound$k_{H}^{\Theta}\left\lbrack \frac{M}{atm} \right\rbrack$ Methanol 2.2 ·10² CH₃OH Acetone 3.1 · 10¹ CH₃COCH₃ Dimethyl Sulfide 9 · 10⁻¹ CH₃SCH₃α-Pinene 4.9 · 10⁻² C₁₀H₁₆ Butane 1.2 · 10⁻³ C₄H₁₀ Sulfur Dioxide 1.2 ·10¹ SO₂

The solubility of a gas in water depends on the temperature. Inreference to standard conditions (T^(Q)=298.15 K), the Henry's constantis denoted as k_(H) ^(θ). Henry's law can be described as a function oftemperature as: $\begin{matrix}{k_{H} = {k_{H}^{\Theta}{\exp\left\lbrack {\frac{{- \Delta_{soln}}H}{R}\left( {\frac{1}{T} - \frac{1}{T^{\Theta}}} \right)} \right\rbrack}}} & (3)\end{matrix}$

-   -   where Δ_(soln)H is the enthalpy of solution.

A quasi two-dimensional model of VOCs absorption into water wasdeveloped. The model is based on the solution of the speciesconservation equations for the gas phase and for the liquid phase. Ityields to the following expression for the destruction and removalefficiency (DRE): $\begin{matrix}{{DRE} = \frac{\left( {1 - \alpha} \right) \cdot {\mathbb{e}}^{- {\beta{({1 - \alpha})}}}}{1 - {\alpha \cdot {\mathbb{e}}^{- {\beta{({1 - \alpha})}}}}}} & (4)\end{matrix}$where β is the ratio between the convection time and the diffusion timeand can be written as: $\begin{matrix}{\beta = {\frac{4\tau_{c}}{\tau_{d}} = \frac{\left( {D/d^{2}} \right)}{\left( {V_{g}/L} \right)}}} & (5)\end{matrix}$with: D=diffusion coefficient in air;

-   -   d=electrode separation distance;    -   V_(g)=velocity of the gas;    -   L=length of the wet corona reactor; and

-   α is a ratio between gas and liquid flow rate that is expressed as:    $\begin{matrix}    {\alpha = \frac{Q_{g}}{Q_{w}V_{mol}k_{H}}} & (6)    \end{matrix}$    with: Q_(g)=gas flow rate;    -   Q_(w)=water flow rate;    -   V_(mol)=gas volume per unit mole.        D and k_(H) are the only parameters related to the specified        compound whose removal is being analyzed; the other factors        describe the geometry of the reactor (d and L) and the        experimental conditions (V_(g), Q_(g) and Q_(w)).

With the assumption, which holds true if wet corona design is madeappropriate, that the convection time is much longer than the diffusiontime, i.e. β>>1, α becomes the critical parameter: for α>1, the DRE isvery low, whereas for α<1 high values of DRE are obtained. Because thegas flow rate is fixed for particular technology stream and V_(mol) andk_(H) are constant in standard conditions, assuming α=α_(crit)=1, acritical value for the water flow rate can be derived as:$\begin{matrix}{Q_{w - {crit}} = \frac{Q_{g}}{V_{mol}k_{H}}} & (7)\end{matrix}$

Only a flow rate higher than this critical value provides a high removalof a given substance by absorption into the water film. This theoreticalanalysis shows good agreement with the experimental results presentedbelow.

Methanol removal by absorption into water was studied in the wet coronareactor. An initial concentration equal to 200 ppm of methanol wasconsidered. Methanol is the most soluble among the organic compounds ofinterest, as Table II shows. The value of the critical water flow ratefor absorbing methanol was calculated from the model to be Q_(w-crit)about 0.2 ml/min, in good agreement with the experimental results shownin FIG. 2. For a fairly low flow rate of the water film (about 0.9ml/min), the DRE is almost complete (about 99.9%), suggesting that anoptimal configuration in terms of the geometry of the reactor and of theratio between gas and liquid flow rate has been obtained for the removalof methanol.

Acetone removal by absorption into water was studied for flow rate ofthe water film varying from zero to about 2 ml/min. The initialconcentration of acetone in the gas stream was 200 ppm. For acetone,with a 1 SLM gas flow rate, the calculated value of the critical waterflow rate is Q_(w-crit) about 1.3 ml/min, higher than for methanol,given the lower solubility in water; experimental results, presented inFIG. 3, show good agreement with the theoretical calculations. A highremoval efficiency (about 99%) can be reached using a fairly low waterflow rate (about 2 ml/min).

Thus, absorption into water of compounds with high Henry's lawconstants, such as methanol and acetone, appears to be a very feasiblesolution given the low water flow rate required, which is, by weight,comparable to the gas flow rate or even lower; absorption iseconomically practical for the removal of these compounds fromindustrial gas streams when water is readily available (as in the caseof the paper industry).

In accordance with a method of the invention, substantially all solubleVOC components, like methanol or acetone, are removed from the stream bywater, and the only one radical OH produced by the pulsed coronadischarge is used for treatment of one molecule of insoluble VOC, likedimethylsulfide.

Molecules of insoluble VOCs are non-polar; this is a reason of theirvery low solubility. After the interaction of OH radical with themolecule of insoluble VOC, this molecule transforms into a new moleculeor radical, which is polar and more soluble. Then this more solublesubstance is removed by the water stream of the scrubber. As a resultnot more than 50 eV per molecule of VOC is spent, and this system isapplicable for the waste stream with higher level of contamination(about 200 ppm of insoluble VOC and any reasonable level of solubleVOC).

For example, in this system, one need only spend 10 W-Hour/m³ for thewaste gas treatment to reach 90% DRE for the case of the next VOCcomposition (Table 3). TABLE 3 Gas composition tested in studyIngredient VOC content (ppm) Dimethyl Sulfide 50 Methanol 200 Acetone200 α-Pinene 50

During this study, the gas flow rate was 1 SLM, and water flow rate was1.5 ml/min.

Usually, the radicals being dissolved in the water produce acidicsolutions (pH <7). Additionally, ozone molecules from the coronadischarge also dissolve in the pulsed corona water flow to make it moreacidic. As a result, the corona device is built from the materials thatare stable to acidic solution: stainless steel, plastics, glass, carbonmaterials, and the like.

One basis for the application of the present invention to exhaust gasesof paper industry or a similar industry is the presence of a huge amountof wastewater in this industry. As a result, a small amount ofwastewater provided from the pulsed corona water flow does not create aproblem; i.e., the pulsed corona wastewater flow can be mixed with theexisting wastewater and the mix can be treated later according to theexisting techniques. If at the particular plant the main wastewater isalkaline (pH >7), an interaction of the pulsed corona wastewater withthe main wastewater stream results in oxidation of the remainingdissolved VOC into H₂O and CO₂; i.e., in the waste mix water cleaning.

If the particular plant has the wastewater that is of such quality thatit can be used in spray systems and does not produce considerable amountof VOCs during spraying, this wastewater can be used in the coronadischarge apparatus.

Currently, the regenerative thermal oxidizers (RTO) are used for VOCtreatment in the paper industry [see, Harkness et al., NCASI TechnicalBulletin No. 795, September (1999)]. These devises are very good for ahigh level of contamination, when VOC oxidation provides a substantialamount of energy. Otherwise, a lot of natural gas should be used for RTOoperation. Other disadvantages of RTO are the SO₂ emission in thetypical case of sulfur-contained VOC and cyclic operation resulting incracking of the ceramic parts of RTO and high capital and operationexpenses. The technology described herein is believed to be economicalreasonable in the case of low energy consumption as can be seen from theannualized cost comparison between the Wet Corona and RTO processesshown in FIG. 4.

Each of the patents, applications, theses and articles cited herein isincorporated by reference. The use of the article “a” or “an” isintended to include one or more.

From the foregoing, it will be observed that numerous modifications andvariations can be effected without departing from the true spirit andscope of the present invention. It is to be understood that nolimitation with respect to the specific examples presented is intendedor should be inferred. The disclosure is intended to cover by theappended claims modifications as fall within the scope of the claims.

1. A method for abatement of volatile organic compounds (VOC) in anexhaust gas stream that comprises passing an exhaust gas stream througha pulsed corona discharge chamber in the presence of a spray of waterdroplets or water film to form one or more oxidation products thatdissolve in the water spray droplets or film and provide an effluentwater stream and an effluent gas stream, wherein (a) said pulsed coronadischarges at a rate of about 0.01 to about 2 kHz, (b) the ratio of thewater spray rate to the exhaust gas flow is about 0.2 to about 2milliliters/minute at one standard liter per minute of exhaust gas flow,and (c) an expenditure of not more than 50 eV per molecule of VOC isutilized, said method providing a destruction and removal efficiency ofabout 90 percent or more, thereby abating said VOC.
 2. The methodaccording to claim 1 wherein said exhaust gas stream is passed through apulsed corona discharge chamber in the presence of a spray of waterdroplets.
 3. The method according to claim 1 wherein said pulsed coronadischarges at a rate of about 0.1 to about 1 kHz.
 4. The methodaccording to claim 1 wherein said exhaust gas stream contains about 60to about 6000 ppm VOC.
 5. The method according to claim 1 wherein saidexhaust gas stream is that produced from a process selected from thegroup consisting of papermaking, metal cleaning or plating, paintmanufacturing, plastics manufacture, petroleum refining and dye-making.6. A method for abatement of volatile organic compounds (VOC) in anexhaust gas stream that comprises passing an exhaust gas stream producedfrom a process selected from the group consisting of papermaking, metalcleaning or plating, paint manufacturing, plastics manufacture,petroleum refining and dye-making and contains about 60 to about 6000ppm VOC through a pulsed corona discharge chamber in the presence of aspray of water droplets to form one or more oxidation products thatdissolve in the water spray droplets and provide an effluent waterstream and an effluent gas stream, wherein (a) said pulsed coronadischarges at a rate of about 0.1 to about 1 kHz, (b) the ratio of thewater spray rate to the exhaust gas flow is about 0.2 to about 2milliliters/minute at one standard liter per minute of exhaust gas flow,and (c) an expenditure of not more than 50 eV per molecule of VOC isutilized, said method providing a destruction and removal efficiency ofabout 90 percent or more, thereby abating said VOC.
 7. The methodaccording to claim 6 wherein said method is carried out at an exhauststream temperature of about 40° C. to about 65° C.
 8. The methodaccording to claim 6 wherein said exhaust stream contains about 200 toabout 4200 ppm VOC.
 9. The method according to claim 6 wherein saidexhaust stream is produced from a papermaking process.
 10. A method forabatement of volatile organic compounds (VOC) in an exhaust gas streamthat comprises passing an exhaust gas stream at a temperature of about40° C. to about 65° C. produced from a papermaking process that containsabout 200 to about 4200 ppm VOC through a pulsed corona dischargechamber in the presence of a spray of water droplets to form one or moreoxidation products that dissolve in the water spray droplets and providean effluent water stream and an effluent gas stream, wherein (a) saidpulsed corona discharges at a rate of about 0.1 to about 1 kHz, (b) theratio of the water spray rate to the exhaust gas flow is about 0.2 toabout 2 milliliters/minute at one standard liter per minute of exhaustgas flow, and (c) an expenditure of not more than 50 eV per molecule ofVOC is utilized, said method providing a destruction and removalefficiency of about 90 percent or more, thereby abating said VOC. 11.The method according to claim 10 wherein said exhaust stream containsabout 300 to about 3000 ppm VOC.
 12. The method according to claim 10wherein said removal efficiency is about 99 percent or more.
 13. Themethod according to claim 10 wherein said papermaking process isbrownstock or oriented strandboard production.
 14. The method accordingto claim 10 including the further step of admixing the effluent waterstream containing oxidized VOC with another waste water stream.
 15. Themethod according to claim 10 wherein the water of said water spray isprovided from waste water of said papermaking process.